Kinetics and Mechanism of Chlorobenzene Degradation in Aqueous Samples Using Advanced Oxidation Processes

Degradation of chlorobenzene using various photoinduced oxidation processes such as direct ultraviolet light-induced photolysis (UV), UV-H2O2, UV-O3, and UV-H2O2-O3 was investigated under aerobic and anaerobic conditions. Kinetics and mechanisms of the degradation process were studied using high performance liquid chromatorgraphy (HPLC) and gas chromatorgraphy-mass spectrometry (GC-MS). In all cases, loss of chlorobenzene followed first-order kinetics. For UV-induced degradation of chlorobenzene, the observed pseudo first-order rate constant, k(obs), ranged from 1.8 x 10(-4) s(-1) under anaerobic conditions to 6.4 x 10(-4) s(-1) for oxygen-saturated solution. Among the four systems studied, under identical conditions, the degradation rates for UV-H2O2 and UV-H2O2-O3 were very similar and were an order of magnitude higher than the one observed for UV. For the UV-H202 system, the observed pseudo first-order rate constant varied linearly with [H2O2] and followed the rate expression k(obs) = kOH[H2O2], where kOH is the observed second-order rate constant for the reaction of OH radical with cholorbenzene. A plot of k(obs) vs. [H2O2] gave a value of 0.17+/-0.02 M(-1) s(-1) for kOH. Both HPLC and GC-MS studies showed that depending upon the time of photolysis and the advanced oxidation processes (AOP) method employed, various intermediates were formed during the degradation process. For the UV process, these intermediates were identified as phenol, biphenyl, chlorobiphenyl isomers, and benzaldehyde. For the other three systems, chlorophenol, and various isomers of chlorobiphenyl and dichlorobiphenyl, were observed as the intermediates. The initial pH of the solution decreased from 5.8 to 3.5, showing the release of chlorine from cholobenzene. The HPLC results also showed that at longer times, the subsequent degradation of the intermediates also took place. Carbon dioxide and water are suspected to be the likely end products. Mechanistic schemes for the formation of such intermediates are proposed.